This invention is directed to the measurement of volumetric fluid flow rate in applications where the flow rate must be accurately determined. In particular, the volume measurement system disclosed herein has been designed to be especially useful in the improved delivery of intravenous (IV) fluids to patients.
The intravenous delivery of fluids to patients is normally carried out with a disposable kit or administration tube set. Such a tube set conventionally comprises a length of flexible rubber or plastic tubing attached at its bottom end to a needle injected into the patient, and connected at its top end to a transparent drip chamber, with the drip chamber being connected through a hard, hollow plastic spike at its upper end to an IV fluid container in the form of a bag or bottle. The transparent drop chamber permits visual determination of the presence of flowing fluid, as well as the ability to sense the drop rate.
The cost of medical care in the United States has steadily increased over recent years. High priced medical equipment required in hospitals has been a major factor in causing rising medical costs. Such equipment includes devices for the intravenous introduction of fluids to hospital patients. Such IV systems fall into the two general categories of gravity feed and pressurized feed. The gravity feed systems rely on gravity to generate the flow of IV fluid from the supply bag or bottle through the drip chamber and tubing to the injection needle. A clamp or other adjustable restrictive device is normally provided on the tubing below the drip chamber, so as to selectively restrict the tubing and adjust the flow rate. Photoelectric sensors, in combination with a light source, have been used adjacent to the drip chamber to sense the drops and to emit a signal used to determine the drop rate. This drop rate may be used by a controller to operate the drive motor of sophisticated clamping devices. These sophisticated clamping devices in the form of motor operated clamps and valve mechanisms are presently used to automatically control the rate of flow through IV tubing by selective restriction of the tubing, and thus of the flow passage.
U.S. Pat. Nos. 3,700,904 and 3,604,419 disclose such electromechanically operated controllers for IV tubing. The latter patent discloses a closed loop servo mechanism for an electromechanical controller in combination with a drop rate detector. U.S. Pat. No. 4,191,184 discloses an IV controller with particularly designed valve members actuated in response to variations in drop rate of the IV fluid. U.S. Pat. No. 3,450,153 discloses a drop rate sensing device which detects the timing of each individual drop pulse and controls the IV fluid rate in response to a detected timing error, in contrast with systems which control flow rate by way of drop pulse count averaging.
U.S. Pat. No. 4,493,710 discloses an electromechanical controller which utilizes specially designed clamping members to place the IV tubing in tension as a means of restricting the internal flow passage within the IV tubing. The clamping members are moved by a drive motor which is activated by a controlling device responsive to signals generated by a drop rate sensor.
In addition to the use of controllers of the aforesaid type on gravity feed systems, infusion pumps have been developed for providing pressurized flow of IV fluid to a patient. Presently there are two types of positive displacement IV pumps; diaphragm/piston and peristaltic. Such pumps are used to accurately maintain IV fluid flow, and are both expensive to purchase and operate. The initial infusion pump price of up to $3,000.00 is soon exceeded by the operating costs of up to $900.00 per year due to the specialized tube sets and equipment used with the pump. There is thus a need for a low cost, high quality infusion system that is also inexpensive to operate.
As with other types of pumps, some peristaltic pumps are volumetric pumps, delivering fluids in measured volumes. Peristaltic pumps accomplish this by crimping the IV tubing of a known inner diameter at two points, trapping and dispensing a known volume of fluid. Peristaltic pumps are noninvasive with respect to the administration set, and are frequently employed. U.S. Pat. Nos. 4,217,993 and 4,213,454 disclose peristaltic pumps with a restriction device downstream of the pumping mechanism. This restriction device provides periodic back pressure against which the pump must operate, so as to help maintain the known inner diameter of the tubing used.
Peristaltic pumps are commonly driven by stepper motors which are powerful enough to force fluids through the IV tubing while maintaining a constant flow rate against the back pressures encountered in the human body. Based on the known inner diameter of a specific pump manufacturer's tube set, the microcontrollers in these pumps are calibrated in terms of a volume output per motor step. An operator simply selects rate of fluid delivery, and the controller determines the stepping rate of the motor. When started, the motor moves at a constant rate, thereby providing a constant flow regardless of back pressure.
Most pumps do not utilize standard IV tube sets because of the errors which such standard tube sets introduce into the volumetric flow. In order to maintain a volumetric accuracy of greater than or equal to 95 percent, IV pumps conventionally utilize special IV tube sets. The tubing for such sets is held to a close tolerance of down to .+-.0.001 inches on its inner diameter of about 0.1 inches, in contrast with tolerances of up to .+-.0.005 or more inches on standard tubing of the same inner diameter. Such greater tolerances guaranty an unacceptable volumetric error of at least plus or minus ten percent from one standard tube set to another. Also, because not all manufacturers use the same inner tubing diameter, and all conventional peristaltic pumps are specifically calibrated for a given manufacturer, all tubing sets are pump and manufacturer specific. Using tube sets from manufacturers other than the pump manufacturer can result in greater than .+-.10% volumetric flow rate errors. Such special tube sets also normally have a particularly designed mid section of tubing in the IV set where the pump operates. This section of tubing is more resilient than the surrounding sections, and its inner diameter is closely controlled as stated. These factors contribute to the high price of specialized tube sets.
Most pump manufacturers do not utilize standard IV tube sets in peristaltic pumps also because such tube sets are less resilient than specialized tube sets. As a result, standard tube sets fatigue faster and do not recover as completely after being crimped as do specialized tube sets. This poor resiliency has the effect of reducing the inner diameter of the tubing over time, which alters the actual output volume and decreases the accuracy of the pumping system. As a result patients may be given too much or not enough medication.
In order to fill the need for a new infusion system with a low operating cost, an inexpensive and accurate method to measure volumetric flow rate through any type of standard IV tubing is needed. A sensing technique and apparatus which measures volumetric fluid flow rate independently of IV tubing inner diameter and wear has been developed to meet that need.
When incorporated into an IV pump or controller this new volumetric flow measurement system will work with any tube set from any manufacturer while maintaining a high volumetric flow rate accuracy. This unique sensing technique is based on determining the volume of fluid drops as they fall through the drip chamber of an IV-type tube set. Drip rate is monitored along with drop volume, so that actual volumetric flow rate may be calculated and used to control the operation of a pump or controller. U.S. Pat. No. 4,105,028 discloses an intravenous fluid delivery system which purports to provide an indication of the size of fluid drops detected by a drop sensor. A clamping type of flow controller periodically impinges the IV tubing at predetermined frequencies in direct response to the size of formed drops. However, drop size alone is used to control the flow rate, and not total flow volume. Drop volume is not computed in this disclosed system, but rather only a minimum size dimension is monitored. Furthermore, the controller disclosed in that patent is invasive to the administration set in that it requires some controlling or sensing surface to pass into the tube set and come in direct contact with the fluid being delivered. A special drip chamber is also required so as to control the formation of drops in response to size variations, thereby increasing the cost of the tube set. This drip chamber is part of a special tube set which must be utilized. The drip chamber incorporates electrodes with which the drops must be in direct contact in order for the system to function as intended.
An article by B. J. Azzopardi, "International Volume of Heat and Mass Transfer," Volume 22, No. 9, September 1979, discloses the use of two drop detectors, one below the other, in combination with light beams and fiber optics to determine the size, velocity, and frequency of fluid drops traveling through the sensor fields. This system apparently requires that the drops be smaller in size than each sensor field, and thus would present difficulties in application to IV systems where relatively large fluid drops are encountered.
U.S. Pat. No. 4,583,975 issued on Apr. 22, 1986 discloses a piezoelectric drop counter and method wherein piezoelectric film strips are used to sense and generate signals in response to the impingement of drops onto a surface of accumulated fluid in a drip chamber.
These disadvantages of previously known drop sensing devices are overcome in the improved drop volume measurement system disclosed herein.